In 1993 Nelson and associates described a "genomic mismatch scanning" (GMS) method to directly identify identical-by-descent (IBD) sequences in yeast (Nelson, S. F., et al., Nature Genetics, 1993, 4:11-18; this and other papers cited herein are expressly incorporated in their entireties by reference). The method allows DNA fragments from IBD regions between two relatives to be isolated based on their ability to form mismatch-free hybrid molecules. The method consists of digesting DNA fragments from two sources with a restriction endonuclease that produces protruding 3'-ends. The protruding 3'-ends provide some protection from exonuclease III (Exo III), which is used in later steps. The two sources are distinguished by methylating the DNA from only one source. Molecules from both sources are denatured and reannealed, resulting in the formation of four types of duplex molecules: homohybrids formed from strands derived from the same source and heterohybrids consisting of DNA strands from different sources. Heterohybrids can either be mismatch-free or contain base-pair mismatches, depending on the extent of identity of homologous regions.
Homohybrids are distinguished from heterohybrids by use of restriction endonucleases that cleave fully methylated or unmethylated GATC sites. Homohybrids are cleaved into smaller duplex molecules. Heterohybrids containing a mismatch are distinguished from mismatch-free molecules by use of the E. coil methyl-directed mismatch repair system. The combination of three proteins of the methyl-directed mismatch repair system MutS, MutL, and MutH (herein collectively called MutSLH) along with ATP introduce a single-strand nick on the unmethylated strand at GATC sites in duplexes that contain a mismatch. Heterohybrids that do not contain a mismatch are not nicked. All molecules are then subject to digestion by Exo III, which can initiate digestion at a nick, a blunt end, or a recessed 3'-end, to produce single-stranded gaps. Only mismatch-free heterobybrids are not subject to attack by Exo III; all other molecules have single-stranded gaps introduced by the enzyme. Molecules with single-stranded regions are removed by absorption to benzoylated napthoylated DEAE cellulose. The remaining molecules consist of mismatch-free heterohybrids which may represent regions of IBD.
Nelson, et al., used S. cerevisiae hybrids as a model system and showed that sequences shared by two independently generated hybrids from the same parent strains could be identified in many instances. Experiments of this kind are much easier to do in yeast than humans. The yeast genome is 250 times simpler than the human genome, it contains far fewer repetitive sequences, and genomic sequences of two yeast strains differ more than genomes of unrelated humans. It has thus far not been possible to do comparable experiments with human genomic DNA. In order to do so one needs to use methods to reproducibly generate simplified but highly polymorphic representations of the human genome. Pooling techniques based on mathematical principles are also essential to identify IBD sequences as well as other sequences showing AFD.
The human genome is enormously long, at 3.times.10.sup.9 base pairs, and it is far too complex for efficient reannealing of homologous DNA strands after denaturation. The rate of annealing of a mixture of nucleic acid fragments in liquid phase is inversely proportional to their complexity. Efforts have therefore been made to generate simplified representations of the genome for genetic methods based on cross hybridization of homologous sequences from different genomes. The exact degree of simplification of human genomic DNA needed to achieve efficient annealing depends on the conditions of hybridization including total DNA concentration, hybridization buffer, and temperature. In general a 10-100 fold simplification is needed for efficient annealing to occur at high DNA concentrations in high salt aqueous solutions (Lisitsyn, N. A., et al., Science, 1993, 259:946-951).
It would be useful to have superior ways of analyzing human DNA.